Control circuit and control method for turning on a power semiconductor switch

ABSTRACT

A method of turning on a power semiconductor switch includes receiving a first signal that characterizes a switch-on behavior of the power semiconductor switch, and detecting two or more phases of the switch-on behavior of the power semiconductor switch in response to the first signal. The method further includes detecting a peak indicative of a phase transition between the two or more phases, generating a phase signal indicative of the two or more phases, and providing a variable current to a control input of the power semiconductor switch in response to the first signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/698,292, filed on Sep. 7, 2017, which is a continuation of U.S.patent application Ser. No. 15/017,331, filed on Feb. 5, 2016, whichclaims priority to European Patent (EP) Application No. 15155262.7,filed Feb. 16, 2015. U.S. patent application Ser. Nos. 15/698,292 and15/017,331 and EP Application No. 15155262.7 are hereby incorporated byreference.

BACKGROUND INFORMATION

In order to improve the switching behavior of power semiconductorswitches (such as IGBTs for example), control circuits in the prior artuse external resistors. The latter are coupled to the control terminalof the power semiconductor switch during a switch-on process, such thata switch-on current can flow into the control input in order to switchon the power semiconductor switch. In some examples, the externalresistors can have a resistance of 8Ω or more, which can generatecertain losses in specific situations.

SUMMARY OF THE INVENTION

In a first general aspect, a control circuit for turning on a powersemiconductor switch comprises an input which is configured to receive asignal that characterizes the switch-on behavior of the powersemiconductor switch, a variable current source which is configured tosupply a current with a variable level to a control input of the powersemiconductor switch in order to switch on the power semiconductorswitch, wherein the control circuit is configured to control thevariable current source in a closed control loop in response to thesignal that characterizes the switch-on behavior of the powersemiconductor switch.

In a second aspect in accordance with the first aspect, the controlcircuit further comprises a phase detection circuit which is configuredto detect two or more phases in the course of the switch-on process ofthe power semiconductor switch based on the signal that characterizes aswitch-on behavior of the power semiconductor switch, and to generate aphase signal, which indicates which of the two or more phases the powersemiconductor switch is currently running through, wherein the variablecurrent source is configured to supply the current with variable currentstrength to a control input of the power semiconductor switch inresponse to the phase signal in order to switch on the powersemiconductor switch.

In a third general aspect, a control circuit for turning on a powersemiconductor switch comprises an input which is configured to receive asignal that characterizes the switch-on behavior of the powersemiconductor switch, a phase detection circuit which is configured todetect two or more phases in the course of the switch-on process of thepower semiconductor switch based on the signal that characterizes aswitch-on behavior of the power semiconductor switch, and to generate aphase signal, which indicates which of the two or more phases the powersemiconductor switch is currently running through, and a variablecurrent source which is configured to supply a current with variablecurrent strength to a control input of the power semiconductor switch inresponse to the phase signal in order to switch on the powersemiconductor switch.

The use of a variable current source enables the control circuit to havea variable transconductance. That can be helpful in order to optimizevarious parameters of the circuit (for example in order to reduce lossesduring the switch-on process). The same applies to the control of thevariable current source in a closed control loop. Furthermore, in someexamples, a variable current source can have a higher stability withrespect to production-dictated fluctuations and temperature fluctuationsthan a circuit having external resistors. The detection of phases on thebasis of a signal that characterizes the switch-on behavior of the powersemiconductor switch can be carried out with relatively low circuitryoutlay (particularly if a control input voltage is used). That canreduce the complexity and thus the price of the control circuit.Furthermore, in some examples, the control circuits comprising a phasedetection circuit can be adapted to different power semiconductorswitches in a relatively simple manner. Moreover, in some examples inthe case of power semiconductor switches having medium or high gatecharge the response time can be shortened.

In a fourth aspect in accordance with the second or third aspect, thesignal that characterizes a switch-on behavior of the powersemiconductor switch is a control input voltage which is present at thecontrol input of the power semiconductor switch.

In a fifth aspect in accordance with the fourth aspect, the controlinput voltage is a base-emitter voltage or a gate-source voltage.

In a sixth aspect in accordance with any of the second to fifth aspects,the phase detection circuit identifies the two or more phases in thecourse of a switch-on process on the basis of characteristic features ofthe signal that characterizes a switch-on behavior of the powersemiconductor switch.

In a seventh aspect in accordance with any of the second to sixthaspects, the phase detection circuit contains one or a plurality ofcomparators, wherein each of the one or the plurality of comparators isconfigured to compare the signal that characterizes a switch-on behaviorof the power semiconductor switch with one of one or more referencesignals.

In an eighth aspect in accordance with the seventh aspect, the phasedetection circuit detects transitions between one or a plurality of thephases in the course of a switch-on process of the power semiconductorswitch if the signal that characterizes a switch-on behavior of thepower semiconductor switch exceeds a respective reference signal.

In a ninth aspect in accordance with any of the second to eighthaspects, the phase detection circuit comprises a peak timing detectioncircuit which is configured to identify a point in time of a peak in thesignal that characterizes a switch-on behavior of the powersemiconductor switch.

In a tenth aspect in accordance with the ninth aspect, the phasedetection circuit is configured to detect a transition between a firstand a second phase if the signal that characterizes the switch-onbehavior of the power semiconductor switch reaches the peak.

In an eleventh aspect in accordance with any of the second to tenthaspects, the phase detection circuit is configured to detect at leastfour phases in the course of the switch-on process of the powersemiconductor switch.

In a twelfth aspect in accordance with the eleventh aspect, a firstphase begins if a switch-on signal of a control circuit of the powersemiconductor switch indicates that the power semiconductor switch isintended to be switched on, and wherein the first phase ends if thesignal that characterizes the switch-on behavior of the powersemiconductor switch exceeds a first threshold value.

In a thirteenth aspect in accordance with the twelfth aspect, a secondphase begins if the signal that characterizes the switch-on behavior ofthe power semiconductor switch exceeds the first threshold value, andwherein the second phase ends if the signal that characterizes theswitch-on behavior of the power semiconductor switch reaches a peak.

In a fourteenth aspect in accordance with the thirteenth aspect, a thirdphase begins if the signal that characterizes the switch-on behavior ofthe power semiconductor switch reaches the peak and wherein the thirdphase ends if the signal that characterizes the switch-on behavior ofthe power semiconductor switch exceeds a second threshold value, whereinthe second threshold value is higher than the first threshold value.

In a fifteenth aspect in accordance with the fourteenth aspect, a fourthphase begins if the signal that characterizes the switch-on behavior ofthe power semiconductor switch exceeds the second threshold value.

In a sixteenth aspect in accordance with any of the second to fifteenthaspects, a first phase begins if a switch-on signal of a control circuitof the power semiconductor switch indicates that the power semiconductorswitch is intended to be switched on.

In a seventeenth aspect in accordance with any of the second tosixteenth aspects, a second phase begins if the power semiconductorswitch begins to conduct.

In an eighteenth aspect in accordance with any of the second toeleventh, sixteenth or seventeenth aspects, a third phase begins if anoperating current through the power semiconductor switch reaches a peak.

In a nineteenth aspect in accordance with any of the second to eleventh,sixteenth, seventeenth or eighteenth aspects, a fourth phase begins ifthe power semiconductor switch enters the active region.

In a twentieth aspect in accordance with any of the second to nineteenthaspects, the variable current source can supply a plurality of discretelevels of current strengths.

In a twenty-first aspect in accordance with any of the second totwentieth aspects, the variable current source comprises a plurality ofparallel driver stages.

In a twenty-second aspect in accordance with the twenty-first aspect,each of the plurality of driver stages is configured to supply apredetermined current to the control input of the power semiconductorswitch.

In a twenty-third aspect in accordance with the twenty-second aspect,each of the plurality of driver stages is configured to amplify apredetermined input current by a specific factor.

In a twenty-fourth aspect in accordance with the twenty-third aspect,the control circuit comprises a region having a higher voltage level anda region having a lower voltage level, wherein the input current issupplied by a current source in the region having a lower voltage level,and wherein the predetermined input current is fed without additionallevel shifters into the region having a higher voltage level.

In a twenty-fifth aspect in accordance with any of the twenty-first totwenty-fourth aspects, the control circuit furthermore comprises aselection circuit which is configured to select one or a plurality ofdriver stages that supply the predetermined input current in response tothe phase detection signal.

In a twenty-sixth aspect in accordance with any of the twenty-first totwenty-fifth aspects, the driver stages comprise current mirrorcircuits.

In a twenty-seventh aspect in accordance with the twenty-sixth aspect,the current mirror circuits in each driver stage comprise cascodedcircuits.

In a twenty-eighth aspect in accordance with any of the second totwenty-seventh aspects, the variable current source can generate currentwith at least four different levels of the input current.

In a twenty-ninth aspect in accordance with the twenty-eighth aspect,the current which is supplied to the control input of the powersemiconductor switch has a first level in a first phase in the course ofthe switch-on process of the power semiconductor switch, a second levelin the course of a second phase in the course of the switch-on processof the power semiconductor switch, and a third level in a third phase inthe course of the switch-on process of the power semiconductor switch,wherein the third level is lower than the first level and higher thanthe second level.

In a thirtieth aspect in accordance with the twenty-ninth aspect, thecurrent is reduced in steps from the first to the second level.

In a thirty-first aspect in accordance with the twenty-ninth orthirtieth aspect, the current is set, in a fourth phase, to apredetermined minimum current that just suffices to keep the powersemiconductor switch in a switched-on state.

In a thirty-second aspect in accordance with any of the second tothirty-first aspects, the phase detection circuit comprises one or aplurality of level shifters which are configured to convert an internalsignal that is output by one or a plurality of comparators from a highvoltage level in the control circuit to a low voltage level in thecontrol circuit.

In a thirty-third aspect in accordance with the second or third aspect,the signal that characterizes a switch-on behavior of the powersemiconductor switch is a voltage that is present across the powerterminals of the power semiconductor switch.

In a thirty-fourth aspect in accordance with the thirty-third aspect,the voltage is a collector-emitter voltage or a drain-source voltage.

In a thirty-fifth aspect in accordance with any of the second tothirty-third aspects, the control circuit comprises a charge pumpcircuit and a bootstrap circuit.

In a thirty-sixth aspect in accordance with any of the precedingaspects, the power semiconductor switch is an IGBT.

In a thirty-seventh aspect in accordance with any of the precedingaspects, the control circuit identifies a transition between two phasesof the switch-on process if the signal that characterizes the switch-onbehavior of the power semiconductor switch assumes a maximum.

In a thirty-eighth aspect in accordance with the thirty-seventh aspect,the circuit for detecting the maximum comprises a plurality of delaycircuits, the delays of which are adapted to a respective powersemiconductor switch.

In a thirty-ninth aspect in accordance with the second or third aspect,the two or more phases in the course of a switch-on process of the powersemiconductor switch are detected based on a control input voltage whichis present at the control input of the power semiconductor switch, andadditionally based on a voltage which is present across the powerterminals of the power semiconductor switch.

In a fortieth aspect in accordance with any of the preceding aspects,the control circuit is configured to the effect that a transconductanceof the control circuit can be altered in the course of the switch-onprocess of the power semiconductor switch.

In a forty-first aspect in accordance with any of the preceding aspects,the control circuit is configured such that no external resistors arerequired for switching on the power semiconductor switch.

In a forty-second aspect, a method for switching on a powersemiconductor switch comprises receiving a signal that characterizes theswitch-on behavior of the power semiconductor switch, controlling acurrent to a control input of a power semiconductor switch in a closedcontrol loop based on the signal that characterizes a switch-on behaviorof the power semiconductor switch in order to switch on the powersemiconductor switch.

In a forty-third aspect in accordance with the forty-second aspect, themethod furthermore comprises detecting two or more phases in the signalthat characterizes a switch-on behavior of the power semiconductorswitch, wherein controlling a current to a control input of a powersemiconductor switch comprises varying the current in response todetecting two or more phases in the signal that characterizes aswitch-on behavior of the power semiconductor switch.

In a forty-fourth aspect in accordance with any of the third tothirty-ninth aspects and the forty-second aspect, the control circuit isconfigured to control the variable current source in a closed controlloop in response to the signal that characterizes the switch-on behaviorof the power semiconductor switch.

BRIEF DESCRIPTION OF THE DRAWINGS

Non-limiting and non-exhaustive exemplary embodiments of the inventionare described with reference to the following figures, wherein identicalreference signs refer to identical components in the different figures,unless specified otherwise.

FIG. 1 shows an exemplary control circuit comprising a variable currentsource and a phase detection circuit.

FIG. 2 shows exemplary signal profiles in an IGBT driver comprising acontrol circuit comprising a variable current source and a phasedetection circuit.

FIG. 3 shows an exemplary variable current source and a phase detectioncircuit.

FIG. 4 shows a state diagram of an exemplary control circuit.

FIG. 5 shows an exemplary phase detection circuit which uses acollector-emitter voltage of a power semiconductor switch for phasedetection.

DETAILED DESCRIPTION

The following description presents numerous details for enabling athorough understanding of the present invention. It is clear to theperson skilled in the art, however, that the specific details are notnecessary to implement the present invention. Elsewhere, known devicesand methods are not set out in detail, in order not to unnecessarilyhamper understanding of the present invention.

In the present description, a reference to “one embodiment”, “oneconfiguration”, “one example” or “example” means that a specificfeature, a structure or property, which is described in conjunction withthis embodiment is included in at least one embodiment of the presentinvention. In this regard, the phases “in one embodiment”, “one example”or “in one example” at different points in this description do notnecessarily all relate to the same embodiment or the same example.Furthermore, the specific features, structures or properties can becombined in arbitrary suitable combinations and/or subcombinations inone or more embodiments or examples. Special features, structures orproperties can be included in an integrated circuit, in an electroniccircuit, in a circuit logic or in other suitable components whichprovide the described functionality. Furthermore, it is pointed out thatthe drawings serve the purpose of elucidation for the person skilled inthe art and that the drawings are not necessarily drawn true to scale.

Firstly, a schematic construction of an exemplary control circuit isexplained with reference to FIG. 1. Exemplary signal profiles in acircuit comprising an exemplary control circuit are then discussed inassociation with FIG. 2. Exemplary configurations and optionalcomponents of the control circuit are discussed hereinafter. FIG. 3 andFIG. 5 show such exemplary configurations.

Firstly, the functions of the elements of an exemplary control circuit100 will be discussed with reference to FIG. 1. FIG. 1 shows anexemplary control circuit 100 for a power semiconductor switch 108. Thecontrol circuit 100 in FIG. 1 is configured to control a switch-onprocess of the power semiconductor switch 108 by supplying a variablecurrent to the control input of the semiconductor switch 108. Onlycontrol of the switch-on process is discussed hereinafter. However, thecontrol circuits disclosed herein can also be used for controlling theturn-off process of a power semiconductor switch (particularly controlcircuits which comprise a variable current source and a phase detectioncircuit). In general, a turn-off process leads a semiconductor switchfrom a switched-on state (“ON state”) to a switched-off state (OFFstate). In this case, a current flows in the turned-on state, while nocurrent flows in the switched-off state.

As can be seen in FIG. 1, the power semiconductor switch 108 has acontrol input G, and two further inputs C, E, wherein the currentthrough and/or the voltage between the further inputs C, E is controlledby a signal at the control input G.

The devices and methods are elucidated below on the basis of the exampleof IGBTs. However, the control circuits and control methods are notrestricted to use with IGBTs. Rather, they can also be used incombination with other power semiconductor switches. For example, it ispossible to use metal oxide semiconductor field effect transistors(MOSFETs), bipolar transistors, IEGTs (“injection enhancement gatetransistors”) and GTOs (“gate turn-off thyristors”) with the controlcircuits. Moreover, the devices for detecting a profile of a voltageacross a power semiconductor switch, the control circuits and thedevices for providing electrical energy can be used with powersemiconductor switches based on gallium nitride (GaN) semiconductors orsilicon carbide (SiC) semiconductors.

A maximum nominal collector-emitter, anode-cathode or drain-sourcevoltage of a power semiconductor switch in the switched-off state can bemore than 500 V, preferably more than 2 kV.

Moreover, the control circuits are not restricted to power semiconductorswitches. In this regard, it is also possible to use other semiconductorswitches with the control circuits. The effects and advantages which arediscussed here also occur at least in part in systems comprising othersemiconductor switches.

Since IGBTs are discussed below, the terminals of the powersemiconductor switch are designated as “collector”, “gate” and“emitter”. As already explained above, however, the devices and methodsare not restricted to IGBTs. In order to avoid being unnecessarilydrawn-out, the designation “emitter” herein also encompasses theterminal of corresponding power semiconductor switches which isdesignated by “source” or “cathode”. Equally, the term “collector”herein also encompasses the terminal of corresponding powersemiconductor switches which is designated by “drain” or “anode”, andthe term “gate” encompasses the terminal of corresponding powersemiconductor switches which is designated by “base”. The term“collector-emitter voltage” below also encompasses a “drain-sourcevoltage” and a “cathode-anode voltage”, and the terms “collectorvoltage” and “emitter voltage” also encompass a “drain voltage” or“anode voltage” and respectively a “source voltage” or “cathodevoltage”.

The control circuit in FIG. 1 comprises a variable current source 102,104 and a phase detection circuit 118. The variable current source 102,104 is configured to apply a variable current (I_(G)) 106 to the controlinput G of the power semiconductor switch 108 in response to a phasesignal (U_(PS)) 120 in order to switch on the power semiconductor switch108. In this case, the variable current (I_(G)) 106 can assume two ormore (for example more than five) discrete values in the course of theswitch-on process (an exemplary profile of the variable current is shownin the fourth curve from the top in FIG. 2). In other examples, thevariable current source 102, 104 continuously varies the variablecurrent (I_(G)) 106.

In the example in FIG. 1, the variable current source 102, 104 comprisesa current source circuit 102 and a plurality of semiconductor switches(Q₁) 104. The power terminals of the plurality of semiconductor switches(Q₁) 104 are connected in parallel between a first reference voltage(V1) 124 and the control terminal G of the semiconductor switch 108.Consequently, the plurality of semiconductor switches (Q1) 104 canconduct a variable current into the control terminal G of the powersemiconductor switch. By way of example, only a portion of the pluralityof the semiconductor switches 104 can be in an ON state and thus conducta predetermined current into the control terminal G. Alternatively oradditionally, the semiconductor switches can be chosen to carry currentsof different magnitudes (for example by means of a variable dimensioningof the semiconductor switches). In the example in FIG. 1, the pluralityof semiconductor switches 104 are MOSFETs. Other semiconductor switchescan be chosen in other examples.

Besides the plurality of semiconductor switches 104, the variablecurrent source in FIG. 1 comprises the current source circuit 102. Saidcurrent source circuit 102 is configured to receive the phase signal(U_(PS)) 120 and, in response to this signal, to switch the plurality ofsemiconductor switches 104 such that a current having a specific levelis supplied to the control terminal G depending on the respective phaseof the switch-on process of the power semiconductor switch. An exemplaryvariable current source 102, 104 will be discussed further below inassociation with FIG. 3.

The phase signal (U_(PS)) 120 is generated by the phase detectioncircuit 118. In the example in FIG. 1, the phase detection circuit 118receives the control voltage (V_(G)) 114 present at the control input Gof the power semiconductor switch 108. On the basis of the profile ofsaid voltage, the phase detection circuit 118 can determine which phaseof the switch-on process the power semiconductor switch is in andgenerate a corresponding phase signal (U_(PS)) 120. The use of thecontrol voltage (V_(G)) 114 (that is to say the gate-emitter voltage inthe case of an IGBT) for detecting the phase of the switch-on processcan afford advantages in some circuits. Firstly, the control voltagecontains information regarding the phase of the switch-on process. Inaddition, control voltage can be detected more easily than other signalsin the control circuit. In this regard, for example, a high voltage (forexample the collector-emitter voltage) would first have to be brought toa lower voltage level, under certain circumstances. That may beassociated with a certain outlay in terms of circuitry. The same appliesto an operating current (for example a collector-emitter current) of thepower semiconductor switch. Nevertheless, in other examples, it is alsopossible to use a voltage across the high-voltage inputs (for examplethe collector-emitter voltage) or an operating current (for example acollector-emitter current) as detection signal for the phase detection.These signals also contain the required information regarding the phasesof the switch-on process.

The phase detection circuit 118 can furthermore comprise an input whichcan receive a control signal (U_(CMD)) 116 for the power semiconductorswitch. Said control signal (U_(CMD)) 116 can have for example a firstlevel if the power semiconductor switch is intended to be turned on, anda second level if the power semiconductor switch is intended to beswitched off. Consequently, an edge in the control signal (U_(CMD)) 116can signal that the power semiconductor switch is intended to beswitched. The phase detection circuit 118 can detect, on the basis ofthe control signal (U_(CMD)) 116, when a switch-on process of the powersemiconductor switch begins. This instant may simultaneously be thebeginning of a first phase of the switch-on process (where the variablecurrent source supplies a current having a first level to the controlinput G).

The control circuit in FIG. 1 forms a closed control loop forcontrolling the switch-on process of the power semiconductor switch 108.The controlled variable is the current supplied to the control input G.The measurement variable is the voltage at the control input (V_(G)) inthe case of FIG. 1. A present phase of the switch-on process isdetermined from said control voltage. Depending on the phase which thepower semiconductor switch is currently in, the control circuit in turnchooses a fixed or variable current. This respective current is suppliedto the control input of the power semiconductor switch (which “closes”the control loop). Many control circuits in the prior art use controlwithout feedback (i.e. an open control loop). The control circuit inFIG. 1 can achieve better results in comparison with those circuits,since a present state of the power semiconductor switch can influencethe switch-on process. Moreover, the control in FIG. 1 can managewithout external control terminal resistors.

Now that an exemplary control circuit has been presented in associationwith FIG. 1, an explanation will be given below, with reference to thecurves in FIG. 2, concerning those phases of the switch-on process ofthe power semiconductor switch which the control circuit detects (andvaries the current to the control input of the power semiconductorswitch in response to the detection).

FIG. 2 shows five idealized and exemplary curves. The topmost curve 202shows a collector-emitter current (I_(CE)) of a power semiconductorswitch, and the second curve 204 shows a collector-emitter voltage(V_(CE)) 204. The profile of a gate voltage (V_(G)) (i.e. the voltage atthe control terminal of the IGBT) is plotted schematically in the thirdcurve 214. The fourth curve 206 illustrates an exemplary profile of acurrent (I_(G)) which is fed from the variable current source into thecontrol input of the power semiconductor switch. Finally, in thebottommost curve 216 there is an exemplary control signal (U_(CMD)) forthe switching of the power semiconductor switch.

As plotted in FIG. 2, the switch-on process of the power semiconductorswitch can be divided into different phases (A0, A, B and C). The powersemiconductor switch is in a phase in each case for a specific timeduration. The power semiconductor switch has a specific state in each ofsaid phases. The length and manifestation of the phases depend, interalia, on the parameters of the power semiconductor switch, theparameters of the control circuit and the load and also the operatingparameters. As already described above, the control circuits describedherein are configured to identify a present phase of the switch-onprocess and to adapt the variable control current on the basis of thisidentification.

The curves shown in FIG. 2 illustrate the switch-on behavior of an IGBT.However, other semiconductor switches also exhibit an at least partlysimilar switch-on behavior. In this regard, different phases can also beidentified in the switch-on behavior of a power MOSFET or of a powerbipolar transistor. Consequently, the control circuits described hereincan also be used for other power semiconductor switches.

Furthermore, it is not mandatory for the control circuit to detect thefour phases A0, A, B, C shown in FIG. 2 and to vary the current inresponse to the detection. Rather, in some examples, it is also possibleto detect only a selection (for example two or three) of the phasesshown in FIG. 2. In further examples, the division of the phases candeviate from the division shown in FIG. 2. In this regard, in somecontrol circuits, a transition between a first and a second phase canoccur at a different point than that shown in FIG. 2. In this example,too, however, a control circuit described herein can vary a current,induced into the control input of the power semiconductor switch, inresponse to the respectively present phase of the switch-on process.

The exemplary profile of the control signals of a control circuit asshown in FIG. 2 will now be discussed, which control circuit varies thecurrent to the control input (I_(G)) in a closed control loop on thebasis of the detected phases of the switch-on process of a powersemiconductor switch, in order to switch on the power semiconductorswitch.

The switch-on process begins at the instant t0 with a state change ofthe control signal U_(CMD) (from a low to a high voltage level in theexample in FIG. 2). A control unit of the power semiconductor switchthereby signals that said power semiconductor switch is to be switchedon. The phase detection circuit can detect this state change in thecontrol signal (U_(CMD)). A first phase A0 of the switch-on processbegins with the state change. In the example in FIG. 2, in the phase A0a constant current (I_(G)) is introduced into the control input of thepower semiconductor switch (as can be seen in curve 206).

In the first phase, the power semiconductor switch does not yet conduct(the collector-emitter current (I_(CE)) is approximately zero).Therefore, the collector-emitter voltage (V_(CE)) remains at its (high)level which it has during the switched-off state of the powersemiconductor switch. The profile of the control input voltage (V_(G))(gate-emitter voltage) is determined by a charging process of differentcapacitances in the power semiconductor switch. By way of example, inthe case of an IGBT, different capacitances can occur between gate andsource of the MOSFET control head (for example an oxide capacitance, acapacitance of the depletion zone, a capacitance between a gateelectrode and the emitter electrode, and so on). These capacitances arecharged by the current applied to the control input in the phase A0. Thevoltage at the control input of the power semiconductor switchsubsequently rises with a time constant determined by the internalcapacitances of the power semiconductor switch.

A second phase of the switch-on process of the power semiconductorswitch begins at the instant t1 at which the IGBT begins to conduct.That occurs if the control voltage VG reaches a threshold voltage of theMOSFET of the IGBT. The collector-emitter current (I_(CE)) then risesgreatly (in this phase, a high collector-emitter voltage (V_(CE)) ispresent at the IGBT; therefore, the rise in the collector-emittercurrent (I_(CE)) is determined by the current gradient in the saturationregion of the IGBT). The collector-emitter voltage (V_(CE)) does not yetdecrease greatly (e.g. since a freewheeling diode of the IGBT cannot yettake up voltage). The control input voltage (V_(G)) furthermoreincreases at a rate determined by the capacitances of the IGBT.

The phase detection circuit can detect the beginning of the second phaseA on the basis of the control input voltage (V_(G)). For example, thephase detection circuit can be configured to detect the beginning of thesecond phase A if the control input voltage (V_(G)) exceeds apredetermined threshold value. Some examples in this respect arepresented further below. As already mentioned, the phase detection canalso take place on the basis of other signals. In this regard, as can beseen in FIG. 2, the characteristic of the collector-emitter voltage(V_(CE)) and of the collector-emitter current (I_(CE)) changes at theinstant t1. Consequently, in other examples, a phase detection circuitcan identify the beginning of the second phase A on the basis of thecollector-emitter voltage (V_(CE)) or the collector-emitter current(I_(CE)).

In response to a detection of the beginning of the second phase A, thevariable current source of the control circuit varies a level of thecurrent which is conducted into the control input of the powersemiconductor switch. In the example in FIG. 2, the current which isconducted into the control input of the power semiconductor switch isreduced from its value in the first phase A0 in stages (for example inthree or four stages). The length of the individual stages can bepredetermined and adapted to the respective power semiconductor switch.For this purpose, the control circuit can define a predetermined set ofdelays that can be chosen by the user. Alternatively, a length of theindividual phases can be set in response to the reaching of thresholdvoltages by the control input voltage (V_(G)). In some examples, acurrent level in a last subphase of the second phase A is chosen tocorrespond to a nominal energy of the power semiconductor switch. InFIG. 2, the current which is conducted into the control input of thepower semiconductor switch is reduced in stages. In one example, thelength of the stages can be chosen such that a transition to the lastsubphase of the second phase A takes place during normal operation ifthe load current through the power semiconductor switch reaches a levelcorresponding to a level of the load current during the Miller plateauof the switch-on process. In other examples, the current can also bereduced continuously.

The reduction in stages—as shown in FIG. 2—of the current which isconducted into the control input of the power semiconductor switch canbring about an increase in the current gradient of the collector-emittercurrent (I_(CE)) of the power semiconductor switch. As a consequencethereof, a maximum value of the collector-emitter current (I_(CE)) isreached earlier than in circuits which do not use the reduction of thecontrol input current in stages. That can have the effect of reducing anenergy loss during the switch-on process.

A third phase B of the switch-on process begins if the collector-emittercurrent (I_(CE)) of the power semiconductor switch reaches its maximumvalue (at the instant t2 in FIG. 2). Afterward, the freewheeling diodeof the IGBT takes up voltage, the collector-emitter voltage (V_(CE))decreases accordingly and the collector-emitter current (I_(CE)) fallsto a constant load current value. As can be discerned in FIG. 2, thebeginning of the third phase can also be detected on the basis of thecollector-emitter voltage (V_(CE)) or the collector-emitter current(I_(CE)).

The phase detection circuit of the control circuit can detect thebeginning of the third phase in response to the control input voltage(V_(G)) reaching the peak. In response to the identification of thebeginning of the third phase B, the control circuit again varies thelevel of the current (I_(G)) introduced into the control input. In oneexample, the control circuit increases the level of the current (I_(G))to a value between the value from the last subphase of the second phaseA and the value (or the smallest of the values) of the first phase A0.The current (I_(G)) that flows into the control input discharges aninternal capacitance of the power semiconductor switch in the thirdphase B. Owing to the rapid fall in the collector-emitter voltage(V_(CE)) and owing to the increase in the internal capacitances of thepower semiconductor switch, the control input voltage (V_(G)) has asubstantially flat profile in this phase.

As shown in FIG. 2, the control circuit can also vary the current(I_(G)) introduced into the control input in different subphases in thethird phase B. In this regard, in the example from FIG. 2, the current(I_(G)) firstly has a constant value before a rising control inputvoltage (V_(G)) is accompanied by a falling current (I_(G)) at theinstant t3.

A fourth and last phase C of the switch-on process begins at the instantt4. The collector-emitter voltage (V_(CE)) has fallen to such a greatextent that the IGBT reaches its active range. The collector-emittercurrent (I_(CE)) has reached its nominal value in the switched-on state.The control input voltage (V_(G)) strives towards its steady-statevalue. The beginning of the fourth phase C can be identified by thephase detection circuit if the control input voltage (V_(G)) exceeds afurther (second) predetermined threshold value.

In response to the detection of the beginning of the fourth phase C, thecontrol circuit can again vary a level of the current (I_(G)) which isconducted into the control input. In the example from FIG. 2, thecontrol circuit sets the current (I_(G)) to a constant level. The latteris lower than the levels of the variable current (I_(G)) in allpreceding phases A0, A, B. In one example, the current (I_(G)) is chosensuch that it corresponds to a minimum current (if appropriate takingaccount of a predetermined safety margin) to keep the IGBT in operation.This choice can be advantageous with the use of some topologies of thevariable current source, since these circuits consume power during theentire switched-on duration of the IGBT. A lowest possible level of thecurrent (I_(G)) can thus reduce the power loss of the control circuitduring the switched-on duration of the IGBT.

The last sections have explained how an exemplary control circuit variesa current supplied to the control input in the course of the switch-onprocess of a power semiconductor switch. As already mentioned, thecontrol circuit can also detect only a portion of the four phases inFIG. 2 and correspondingly vary the current (I_(G)) (e.g. only thefirst, second and third phases A0, A, B, only the second, third andfourth phases A, B, C or only the second and fourth phases A, C).

In association with FIG. 2, the last sections have functionallydescribed an exemplary control circuit. The next sections will discussexemplary circuits for implementing the control circuit in associationwith FIGS. 3 to 5.

As already explained with reference to FIG. 1, the control circuit cancomprise a variable current source and a phase detection circuit. Anexemplary configuration of these two components can be seen in FIG. 3.The variable current source 102, 104 comprises a current source circuit102 and a plurality of semiconductor switches (Q₁) 104. The phasedetection circuit 118 comprises various components for detecting whichphase of the switch-on process the power semiconductor switch iscurrently in. Both components (the variable current source 102, 104 andthe phase detection circuit 118) will be explained in more precisedetail below.

The variable current source 102, 104 will be discussed first. As can beseen in FIG. 3, the variable current source 102, 104 has a plurality ofparallel current source stages in one example. Each of said stagescomprises a current amplifier 102 and a semiconductor switch 104, whichcan supply a current to the control input G of the power semiconductorswitch. The output currents of the parallel current source stages can bechosen such that different current levels required by the controlcircuit are available as a result of switching on and off. In this way,a variable current can be applied to the control input G of the powersemiconductor switch during the switch-on process.

In one example, the current amplifiers 102 can be configured to receivea phase signal (U_(PS)) 120 and a reference current signal (U_(CS)).Hereinafter, it is assumed that the reference current signal (U_(CS)) isa reference current. In other examples it may be a reference voltage.The variable current source 102, 104 can activate one or a plurality ofparallel current source stages in response to the phase signal (U_(PS))120. A predetermined current (depending on the output current of the oneor more current source stages) is then conducted into the control input.The current source stages can supply different or identical outputcurrents. Four parallel current source stages are depicted schematicallyin FIG. 3. However, the number of current source stages can be differentin other examples (e.g. two (or more), three or more than four). Thenumber and the output currents of the current source stages can bechosen such that a variable current source can provide the majority ofoutput currents required (by activation of one or a plurality of currentsource stages).

In this case, the phase detection signal can contain information thatthe power semiconductor switch has entered one of the phases discussedabove in association with FIG. 2 (phase A0, A, B or C). The variablecurrent source 102, 104 can supply output currents having the properties(levels and time durations) such as were likewise explained inassociation with FIG. 2.

In order to provide the respective output currents, each of the parallelcurrent source stages can amplify the reference current with apredetermined factor. In one example, the reference current is receivedfrom a region of the control circuit with a lower voltage level (forexample 4 to 6 V). On the other hand, the semiconductor switches 104 ofthe variable current source 102, 104 are connected between the controlinput G of the power semiconductor switch and a reference voltage of aregion of the control circuit having a higher voltage level V1 (forexample 24 to 26 V). Consequently, the semiconductor switches 104conduct the variable output current of the variable current source 102,104 from the region of the control circuit having a higher voltage levelV1 into the control input. In one example, the variable current source102, 104 achieves the above-described conversion from the region havinga low voltage level into the region having a higher voltage levelwithout using level shifters.

In one example, each of the parallel current source stages contains acurrent mirror circuit which amplifies the reference current to therespective output current. In one example, each current mirror circuitcan comprise a plurality of MOSFET semiconductor switches dimensioned toprovide the respective output current. In addition, the semiconductorswitches of one or more of the parallel current source stages can bearranged in a cascoded manner. That can improve an output resistance ofthe current source stages. Additionally or alternatively, thesemiconductor switches in each of the parallel current source stages canbe of different design. In this regard, in one example, it is possibleto use semiconductor switches having a low nominal voltage andsemiconductor switches having a higher nominal voltage.

The use of current mirror circuits for generating the output current inthe variable current source can afford various advantages. Firstly, atransconductance of the circuit can be varied by injection of a desiredcurrent. Moreover, the variable transconductance can make it possible torealize a closed control loop when switching on the power semiconductorswitch. Furthermore, process and temperature fluctuations can be (atleast partly) compensated for in the current mirror circuits. Moreover,a response time of the variable current source can be reduced (forexample in comparison with a circuit having an inverter chain).

As already mentioned, the variable current source 102, 104 can comprisea plurality of semiconductor switches. In one example, the semiconductorswitches can be integrated with a separate source well (in the case ofMOSFETs). That can reduce a variation of the output current with thetemperature.

In other examples, active regions of the semiconductor switches (in thecurrent source stages) can be chosen as integer multiples of a referencesemiconductor switch. That can reduce a fluctuation of the outputcurrent as a result of a process-dictated variation during production.

It has already been explained in association with FIG. 2 that thevariable current source can provide an output current having differentlevels within a phase (see, for example, the second phase A in FIG. 2).In one example, the two or more stages of the output current can begenerated in response to output pulses of a pulse generating circuit.The latter can generate one or more pulses of predetermined lengthbeginning at the instant at which the phase signal (U_(PS)) signals thebeginning of a specific phase. The variable current source can generatea predetermined current starting from a state change of each of thepulses. Consequently, the pulse generating circuit can attain the timeduration of the different subphases without using an oscillator or thelike. A multi-stage configuration of the variable current was discussedonly for the second phase A in FIG. 2. However, this technique (using apulse generating circuit) can also be used in other phases of theswitch-on process.

The preceding sections discussed some examples of the implementation ofthe variable current source 102, 104. The following sections willdiscuss various aspects regarding the implementation of the phasedetection circuit 118, once again on the basis of the example in FIGS. 3and 5. The phases of the switch-on process of the power semiconductorswitch which the phase detection circuit can detect have already beendiscussed in association with FIG. 2.

The phase detection circuit 118 in FIG. 3 contains a state circuit 302,a peak timing detection circuit 304, a plurality of comparators 310,312, 314 and a plurality of optional level shifters 316, 318, 320. Bymeans of these components, the phase detection circuit can determine thephase of the switch-on process which the power semiconductor switch iscurrently in.

In FIG. 3, the phase detection circuit 118 receives the control inputvoltage (V_(G)) and the control signal (U_(CMD)). On the basis of thesesignals, the phase detection circuit 118 can determine which phase ofthe switch-on process is currently being run through. As alreadymentioned, instead of the control input voltage (V_(G)), it is alsopossible to use some other signal that characterizes the switch-onbehavior of the power semiconductor switch (for example thecollector-emitter voltage (V_(CE)) or the collector-emitter current(V_(CE))). As explained further below in association with FIG. 5, inaddition to the control input voltage (V_(G)), it is possible, moreover,to use one or a plurality of further other signals characterizing theswitch-on behavior of the power semiconductor switch (for example thecollector-emitter voltage (V_(CE)) or the collector-emitter current(V_(CE))).

The phase detection circuit generates a phase detection signal (U_(PS))120 which indicates the respective phase (and/or a transition betweentwo phases). In some examples, the phase signal (U_(PS)) 120 can includea plurality of channels each indicating the presence of one specificphase or the instant of a transition between two specific phases. Asdescribed further above, the phase signal (U_(PS)) 120 is received bythe variable current source 102, 104, which outputs a correspondingcurrent in response to the phase signal (U_(PS)) 120.

The phase detection circuit 118 is configured to detect two differentcharacteristics of the control input voltage (V_(G)): firstly, the phasedetection circuit 118 can detect when the control input voltage (V_(G))exceeds (or falls below) one or a plurality of predetermined thresholdvalues. Secondly, the phase detection circuit 118 can detect when thecontrol input voltage (V_(G)) has a peak. It has already been explainedin association with FIG. 2 that these two detection steps can sufficefor identifying a plurality of phase transitions during the switch-onprocess of the semiconductor switch. Firstly, the plurality ofcomparators 310, 312, 314 will now be discussed, with the aid of whichthe phase detection circuit 118 can ascertain when the control inputvoltage (V_(G)) exceeds a specific threshold value. In the example fromFIG. 3, the comparison circuit is configured in the manner of a flashADC (i.e. the plurality of comparators 310, 312, 314 operate inparallel). The control input voltage (V_(G)) is compared respectivelywith a reference voltage TH1, TH2, THN in the parallel comparators 310,312, 314. The phase detection circuit 118 can thus determine when aspecific limit value TH1, TH2, THN is exceeded. In the example in FIG.2, a comparison of the control input voltage (V_(G)) with a thresholdvalue may be necessary for the identification of the phase transitionfrom the first to the second phase (A0→A) and for the identification ofa transition from the third to the fourth phase (B→C). These comparisonsare carried out in each case by one of the plurality of parallelcomparators 310, 312, 314.

In the example from FIG. 3, moreover, a plurality of the parallelcomparators 310, 312, 314 are also used for the detection of a point intime at which the control input voltage (V_(G)) reaches the peak. Thesecomparison values can therefore be used for detecting the transitionfrom the second to the third phase (A→B) in accordance with FIG. 2. Thatis explained in further detail further below in association with thepeak detection circuit 304.

In one example, the reference voltages TH1, TH2, THN can be generatedwith reference to the first reference voltage (V₁). For example, thefirst reference voltage (V₁) can be divided down by one or a pluralityof voltage dividers (e.g. one voltage divider per comparator 310, 312,314) to the desired reference voltages TH1, TH2, THN.

The comparison circuit in the manner of a flash ADC in FIG. 3 may beadvantageous if the control input voltage (V_(G)) is used for detectingthe phases of the input signal, since the comparison circuit in themanner of a flash ADC in FIG. 3 can have a high input impedance. Thecontrol input G can have a high output resistance, which makes it moredifficult to use a detection circuit having a low input resistance(since otherwise charge could flow away from the control input, whichcould disturb the signal to be measured).

In one example, one or a plurality of the comparators 310, 312, 314 areconfigured as folded transconductance amplifier with push-pull outputstage. In a further example, one or a plurality of the comparators 310,312, 314 are configured as latched circuit with push-pull output stage.Both alternatives can have a large common-mode range, which may benecessary for detecting a point in time at which the control inputvoltage (V_(G)) reaches the peak (the voltage at the peak of the controlinput voltage (V_(G)) can be, for example, 96% of the first referencevoltage V₁). The solution with a latched circuit with push-pull outputstage can in this case have a smaller structural size of the outputstage and a higher rate of rise in comparison with the solution with atransconductance amplifier with push-pull output stage.

Since the state circuit 302 is arranged in a region having a lowervoltage in the example from FIG. 3, the output signals of thecomparators 310, 312, 314 are reduced by corresponding level shifters316, 318, 320. The resulting signals CO1, CO2, CON can be received andprocessed further by the state circuit 302 and the peak detectioncircuit 304.

The following sections will now discuss an exemplary peak timingdetection circuit 304 in greater detail.

In one example, the peak timing detection circuit 304 can receive theoutput signals CO_(N), CO_(N-1) . . . CO₃ from a plurality of thecomparators 310, 312, 314 as input signal. The peak timing detectioncircuit 304 can be configured to identify a peak in the control inputvoltage (V_(G)) if the control input voltage (V_(G)) has exceeded allthreshold voltages of a subset of the comparators 310, 312, 314 (forexample one or two comparators) which supply the output signals CO_(N),CO_(N-1) . . . CO₃ and has again fallen below the threshold voltage ofthe comparator of the subset having the highest threshold voltage. Inthis case, the peak timing detection circuit 304 signals to the statecircuit 302 by means of a detection signal (U_(DT)) 306 that the controlinput voltage (V_(G)) has reached a peak. In this example, the peakdetection is carried out independently of a magnitude of the voltagemaximum of the control input voltage (V_(G)).

In some cases, the control input voltage (V_(G)) does not have apronounced peak. Moreover, the peak timing detection circuit presentedabove may be disturbed by oscillations during the transition from thesecond phase A to the third phase B. Therefore, a peak timing detectioncircuit 304 (in addition or as an alternative to the peak timingdetection circuit discussed above) can comprise a further circuit. Anexemplary further circuit is discussed below.

The further exemplary peak timing detection circuit also has the outputsignals of a subset of the comparators 310, 312, 314 as input signals.In addition, a further signal delayed by a predetermined delay isgenerated for each of the subset of the comparators 310, 312, 314. Thedelayed and the undelayed output signals of each comparator 310, 312,314 are applied to an OR gate. In each case the earliest occurringsignal for each threshold voltage is thus determined. The instant of thepeak is determined by an ANDing of the output signals of the OR gate. Inthis case, it is possible to select the individual delays for therespective type of power semiconductor switch (e.g. taking account of agradient of a rise in the control input voltage (V_(G)) after thebeginning of a switch-on process).

The preceding sections explained how the phase detection circuit candetect various features of the control input voltage (V_(G)). Thefollowing sections will explain how the phase which the switch-onprocess is currently running through can be detected from the outputsignals of the comparators 310, 312, 314, the control signal (U_(CMD))116 and the output signal of the peak timing detection circuit 304.

This function is performed by the state circuit 302, the function ofwhich is explained in association with FIG. 4. As can be seen in FIG. 4,the state circuit 302 can comprise a state machine. In the example inFIG. 4, the state circuit 302 can identify which of four phases of theswitch-on process the power semiconductor switch is currently in. Thefour phases correspond to the four phases which were discussed inassociation with FIG. 2 (i.e. phases A0, A, B and C).

As already mentioned, a beginning of the switch-on process can bedetected on the basis of the control signal (U_(CMD)) 116 for thesemiconductor switch. In one example, the beginning of a switch-onprocess of the power semiconductor switch can be detected if the controlsignal (U_(CMD)) 116 has a rising or falling edge. In response to thestate change of the control signal (U_(CMD)) 116, the state circuit 302can identify that the power semiconductor switch has entered the phase402 (A0) of the switch-on process.

Furthermore, the state circuit 302 can decide, on the basis of an outputsignal (CO₁) of a first comparator, whether a transition from the firstphase 402 (A0) to the second phase 404 (A) has taken place. If thecontrol input voltage (V_(G)) exceeds the threshold voltage of the firstcomparator, the output signal CO₁ of the first comparator has a firststate (e.g. a high level CO₁). If the control input voltage (V_(G))falls below the threshold voltage of the first comparator, the outputsignal CO₁ of the first comparator has a second state (e.g. a low levelCO₁ ). In some examples, the comparator must maintain a specific statefor a predetermined time duration before a phase change is detected.

In some examples, the respective comparator (for example the firstcomparator for the transition from phase A0 to phase A) can be turnedoff a predetermined time after the phase transition has been ascertained(e.g. 20 ns to 80 ns). A power consumption of the circuit can be reducedas a result.

As already discussed, a transition to a third phase (B) 408 can beidentified on the basis of an output signal 406 (corresponding to thedetection signal (U_(DT)) 306) of a peak timing detection circuit. Ifsaid output signal 406 signals that the control input voltage (V_(G))has reached a peak (or a maximum), the state circuit 302 changes thepresent phase to the second phase (B) 406. As already mentioned, theoutput signal 406 of the peak detection circuit can determine a point intime of the peak of the control input voltage (V_(G)) on the basis ofthe output signal of a plurality of comparators. The latter can also beturned off at a predetermined time duration (e.g. 20 ns to 80 ns) afterthe identification of the phase transition from the second to the thirdphase (A→B).

The transition from phase B 408 (the third phase) to the phase C 410(the fourth phase) can take place in reaction to the output signal CO2of a second comparator. The state circuit 302 can thus identify whethera transition from the third phase (B) 408 to a fourth phase (C) 410 hastaken place. If the control input voltage (V_(G)) exceeds the thresholdvoltage of the second comparator, the output signal CO₂ of the secondcomparator has a first state (e.g. a high level CO₂). If the controlinput voltage (V_(G)) falls below the threshold voltage of the secondcomparator, the output signal CO2 of the second comparator has a secondstate (e.g. a low level CO₂).

The entry into the fourth phase (C) 410 is the last phase transitionidentified by the state circuit 302. In other examples, the switch-onprocess can be divided into more phases (e.g. five, six or seven or morephases) which can be detected on the basis of the profile of the controlinput voltage (V_(G)). In still other examples, the switch-on processcan be divided into fewer phases (e.g. two or three) which can bedetected on the basis of the profile of the control input voltage(V_(G)). In all cases, it is possible to use phase detection circuitsand variable current sources of the kind as discussed further above forthe case of four phases of the switch-on process.

As soon as the entry into a specific phase has been identified, thestate circuit 302 generates a corresponding phase signal (U_(PS)) 120.This phase signal (U_(PS)) 120 can then be received by a variablecurrent source, as discussed above. The variable current source can inturn conduct an output current (constant or variable) predetermined forthe respective phase to the control input of the power semiconductorswitch, in order to switch on the power semiconductor switch.

The preceding sections discussed exemplary variable current sources andphase detection circuits of the control circuit with reference to FIG. 3and FIG. 4. Another variant of these circuits will be discussed below.

FIG. 5 shows an alternative to the peak timing detection circuit in FIG.3, for identifying the transition from the second phase to the thirdphase (A→B) (i.e. an instant at which a collector-emitter current(I_(CE)) has a maximum value). The phase circuit 118 in FIG. 5 containsa phase transition circuit 504, which receives a signal (V_(CEF)) 536based on the collector-emitter voltage (V_(CE)) as input signal anddetects a phase transition on the basis of this signal.

The signal (V_(CEF)) 536 based on the collector-emitter voltage (V_(CE))is generated by a filter 530 containing a capacitance C1 and a resistiveelement R1, which are coupled between a collector terminal C and anemitter terminal E of the power semiconductor switch.

In one example, a control circuit of an IGBT can comprise a seriescircuit of parallel-connected capacitances and resistive elements, whichis coupled to the collector terminal of the power semiconductor switch(for example in order to provide an active clamping function). Thefilter 530 can be coupled between a last capacitance of the seriescircuit of parallel-connected capacitances and resistive elements and areference voltage level 112.

The filter 530 can be configured to generate a voltage spike if the peakof the collector-emitter current has been exceeded. This signal(V_(CEF)) 536 is forwarded to the phase transition circuit 504. In thephase transition circuit 504, in one example, an instant of the peak ofthe collector-emitter current (I_(CE)) can be determined from the signal(V_(CEF)) 536. This information can be used by the phase transitioncircuit 504 to detect a phase transition from the second phase (A) tothe third phase (B) of the switch-on process. A corresponding detectionsignal (U_(DT)) 506 can in turn be communicated to the state detectioncircuit 502.

In one example, the phase transition circuit 504 can amplify the signal(V_(CEF)) 536 and apply the amplified signal to a differentialamplifier. The resulting signal can have a falling edge at the instantof the peak of the collector-emitter current (I_(CE)) and thus indicatethe phase transition from the second to the third phase (A→B). By way ofexample, the falling edge of the resulting signal can be detected forthis purpose.

In one example, the phase transition circuit 504 can amplify the signal(V_(CEF)) 536 in three amplifier stages. Firstly, a buffer amplifier canreduce the output impedance of the signal to be detected at the V_(CE)node. Afterwards a non-inverting amplification followed by adifferential amplification of the signal can be carried out.

The above description of the illustrated examples of the presentinvention is not meant to be exhaustive or restricted to the examples.While specific embodiments and examples of the invention are describedherein for illustration purposes, various modifications are possible,without departing from the present invention. The specific examples ofvoltage, current, frequency, power, values of ranges, times, etc. aremerely illustrative, and so the present invention can also beimplemented with other values for these variables.

These modifications can be carried out on examples of the invention inlight of the detailed description above. The terms used in the followingclaims should not be interpreted such that the invention is restrictedto the specific embodiments disclosed in the description and the claims.The present description and the figures should be regarded asillustrative and not as restrictive.

What is claimed is:
 1. A method of turning on a power semiconductorswitch, comprising: receiving a first signal that characterizes aswitch-on behavior of the power semiconductor switch; detecting two ormore phases of the switch-on behavior of the power semiconductor switchin response to the first signal; detecting a peak indicative of a phasetransition between the two or more phases; generating a phase signalindicative of the two or more phases; and providing a variable currentto a control input of the power semiconductor switch in response to thefirst signal.
 2. The method of claim 1, wherein the first signal is acontrol input voltage.
 3. The method of claim 2, wherein the controlinput voltage is a base-emitter voltage.
 4. The method of claim 2,wherein the control input voltage is a gate-source voltage.
 5. Themethod of claim 1, wherein detecting a peak indicative of a phasetransition between the two or more phases comprises: using a peak timingdetection circuit to detect the peak in the first signal.
 6. The methodof claim 1, wherein the two or more phases of the switch-on behavior ofthe power semiconductor switch comprises at least four phases of theswitch-on behavior of the power semiconductor switch.
 7. The method ofclaim 1, wherein a first phase begins if a switch-on signal of thecontrol circuit of the power semiconductor switch indicates that thepower semiconductor switch is intended to be switched on, and whereinthe first phase ends if the first signal exceeds a first thresholdvalue.
 8. The method of claim 7, wherein a second phase begins if thefirst signal exceeds the first threshold value, and wherein the secondphase ends if the first signal reaches the peak.
 9. The method of claim8, wherein a third phase begins if the first signal reaches the peak,and wherein the third phase ends if the first signal exceeds a secondthreshold value, wherein the second threshold value is higher than thefirst threshold value.
 10. The method of claim 9, wherein a fourth phasebegins if the first signal exceeds the second threshold value.
 11. Themethod of claim 1, wherein providing a variable current to a controlinput of the power semiconductor switch in response to the first signalcomprises: providing the variable current using a plurality of paralleldriver stages.
 12. A control circuit for turning on a powersemiconductor switch, comprising: an input coupled to receive a firstsignal that characterizes a switch-on behavior of the powersemiconductor switch, wherein the power semiconductor switch comprisesan active clamp; a phase detection circuit coupled to detect two or morephases of the switch-on behavior of the power semiconductor switch inresponse to the first signal, wherein the phase detection circuit iscoupled to generate a phase signal that indicates which of the two ormore phases the power semiconductor switch is currently running through,wherein the phase detection circuit is coupled to detect a peakindicative of a phase transition between the two or more phases; and avariable current source coupled to supply a current with a variablelevel to a control input of the power semiconductor switch to switch onthe power semiconductor switch, wherein the control circuit is coupledto control the variable current source in a closed control loop inresponse to the first signal.
 13. The control circuit of claim 12,wherein the first signal is a base-emitter voltage.
 14. The controlcircuit of claim 12, wherein the first signal is a gate-source voltage.15. The control circuit of claim 12, wherein the phase detection circuitis coupled to detect transitions between the two or more phases of theswitch-on behavior of the power semiconductor switch in response to thefirst signal exceeding a respective reference signal.
 16. The controlcircuit of claim 12, wherein the phase detection circuit is coupled todetect at least four phases of the switch-on behavior of the powersemiconductor switch.
 17. A control circuit for turning on a powersemiconductor switch, comprising: an input coupled to receive a firstsignal that characterizes a switch-on behavior of the powersemiconductor switch, wherein the power semiconductor switch comprisesan active clamp and an insulated gate bipolar transistor (IGBT); a phasedetection circuit coupled to detect two or more phases of the switch-onbehavior of the power semiconductor switch in response to the firstsignal, wherein the phase detection circuit is coupled to generate aphase signal that indicates which of the two or more phases the powersemiconductor switch is currently running through; a variable currentsource coupled to supply a current with a variable level to a controlinput of the power semiconductor switch to switch on the powersemiconductor switch, wherein the control circuit is coupled to controlthe variable current source in a closed control loop in response to thefirst signal; and a filter coupled to an emitter of the IGBT andconfigured to provide a voltage spike indicative of a phase transitionbetween the two or more phases.
 18. The control circuit of claim 17,wherein the active clamp comprises a series circuit ofparallel-connected capacitances and resistive elements coupled to acollector of the IGBT; and wherein the filter is further coupled to theseries circuit of parallel-connected capacitances and resistiveelements.